A group of men face undergoing anal examinations in Tanzania as the African nation's government undertakes a crackdown on homosexual activity.

Amnesty International has condemned the Tanzanian government after it was discovered that ten men had been arrested on suspicion of being gay on the island of Zanzibar. It's understood police received a "tip-off" about a same-sex marriage ceremony.

The men arrested for allegedly conducting the same-sex marriage ceremony are to face a forced anal examination on Friday, activists told the Guardian. The procedure is supposed to discover evidence of homosexual activity.

"This is a shocking blow following the Tanzanian government's assurance that no one would be targeted and arrested because of their actual or perceived sexual orientation and gender identity," said Amnesty International's East Africa deputy director Seif Magango.

"We now fear these men may be subjected to forced anal examination, the government's method of choice for 'proving' same-sex sexual activity among men. This must not be allowed to happen - these men must be released immediately."

The 10 men were arrested when police raided a party at Pongwe Beach, Zanzibar on Saturday night, according to Amnesty International. The men are being held at Chakwa police station in Unguja, even though no charges have been laid.

Police reportedly said they found the men sitting in pairs "two by two", which apparently gave the officers grounds to arrest the men on suspicion of homosexual activity.

"It is mind-boggling that the mere act of sitting in a pair can assume criminal proportions. The police clearly have no grounds to file charges against these men in court, despite arresting them three days ago," said Mr Magango.

The arrests come amid a crackdown on homosexual activity in Tanzania, after Paul Makonda, regional commissioner for Dar es Salaam, Tanzania's biggest city, called for citizens to report homosexuals and sex workers to authorities.

"I have received reports that there are so many homosexuals in our city, and these homosexuals are advertising and selling their services on the internet," Mr Makonda said in a video posted to YouTube recently, which was translated from Swahili by CNN.

"Therefore, I am announcing this to every citizen of Dar es Salaam: If you know any gays ... report them to me," he's understood to have said in the clip.

An LGBT activist told NBC News Mr Makonda said he had received over 18,000 messages since the YouTube video was posted online. He also said up to 200 people had allegedly reported homosexual activity and sex working in the city.

Homosexuality is punishable in Tanzania by a prison sentence of 30 years to life, one of the harshest punishments in the world for same-sex intimacy, according to Human Rights Watch.

The countries in 2016 that practice anal examinations for men suspected of homosexual activity include Tunisia, Egypt, Zambia, Kenya, Uganda and Cameroon.

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